Well, there you go, it worked :)

The return value of 47 seems to indicate 47 messages, and there appear 47 space characters inside $msg -- looks like it worked to me :)

If the messages aren't supposed to be what you get, I'd check if using that syscall works using some ./recvmmsg c-program

If it doesn't, you have your answer, the recvmmsg function is broken, upgrade if you can, or give up

If it does, something goes wrong with perl syscall, so try Inline::C

Good luck figuring it out :)